62 GROWTH AND WORK OF PLANTS 



or root is protected from either drought or cold. The evergreens 

 like the pines, spruces, cedars, the laurels, rhododendrons, etc., 

 have thick leaves which are protected by thick walled epidermal 

 cells, which in turn are also protected by the cuticle, a coating 

 of a waxy substance which largely checks the loss of water. 

 The protoplasm also undergoes certain changes with the on- 

 coming of winter, so that it is more resistant to the injuries 

 accompanying extremely cold weather. 



103. The deciduous trees and shrubs, those which shed 

 their leaves in the autumn and winter, show a very interesting 

 adaptation to meet the rigors of a winter season. The leaves 

 are usually broad and thin, and are especially suited for the 

 purpose of transpiration, that is, the loss of water. By the death 

 and shedding of the leaves, deciduous trees and shrubs get rid 

 of organs, which, if they remained alive and active during the 

 winter, would drain so much water from them that they would 

 dry out and die, since the roots during the cold season absorb 

 but little water from the soil and could not replace that lost 

 through the leaves. The shoots are protected by the matur- 

 ing or ripening of the wood, the cell walls becoming thick and 

 firm. The outer portion of the bark has thick walled cells 

 which are dead and lose most of their water. With each year 

 this bark becomes thicker. This, with the waxy cuticle on the 

 surface, serves to protect the winter shoots. The young growing 

 points, however, consist of delicate masses of cells rich in pro- 

 toplasm and with an abundance of water. If these growing 

 points, or buds, at the ends of the shoots and in the axils of the 

 leaves were unprotected, they would lose a sufficient amount of 

 water during the dry and freezing weather to kill them. But 

 they are protected by coverings in the form of bud scales, hairs, 

 or by both, and often by the abundant formation of a waxy or 

 resinous substance between the scales and on the outside. The 

 bud scales also afford protection to the delicate growing point 

 from mechanical injuries. 



104. The effect of freezing on plant tissue. Very 

 few plants are killed by actual cold or freezing of the tis- i 



